In the prior art, organic and hybrid solar cells have been fabricated with a polychromatic efficiency of 2% and a peak monochromatic efficiency of 7% (Huynh et al., “Hybrid NanoRod-Polymer Solar Cells”, Science 295, 2425-2427 (2002); Schmidt-Mende et al., “Self-Organized Discotic Liquid Crystals for High-Efficiency Organic Photovoltaics”, Science 293, 1119-1122 (2002)). In these solar cell devices, the device architecture is not adequate for that needed for a higher-efficiency device. These devices are limited by the extent to which excitons can be harvested to perform useful work. This is due to three different factors:
First, the nanostructure within the active layer of the solar cells made by other groups were quasi-randomly arranged within a medium of conducting or semiconducting polymer, and, as such, many photons were not converted into excitons. In particular, in the Huynh et al work, since many nanorods were only partially aligned, and large clusters of nanorods (interspersed with areas of few rods) were present in the devices; photonic energy could pass through the device without being incident to an excitonic transformation event. This decreased the efficiency of exciton creation.
Second, of the excitons which did form, many did not reach an electron affinity junction before spontaneously recombining. The lifetime of migrating excitons are extremely short, and as such an exciton can typically travel only 10 nm before the electron and hole spontaneously and non-productively recombine. Thus, to extract the electron away from its bound hole, an exciton must reach a junction of materials with differential electron affinities within 10 nm of the site where the exciton was initially created. In the devices produced by other groups, such as the work of Schmidt-Mende et al. and Huynh et al., the spacing of the nanostructures was quasi-random, and so some areas of the device had many junctions within 10 nm of one another (permitting efficient electron extraction), while many other areas of the device had no junctions at all within 10 nm of one another (resulting in the loss of electrons associated with excitons travelling in those regions of the device). This factor decreased the efficiency of both electron and hole transfer at differential electron affinity junctions.
Finally, the movement of electrons through the active materials of the devices required regularly and closely spaced nanowires or nanorods which could collect and transport free electrons to the outer boundary of the active layer of the device. This factor decreased the hole and electron collection efficiency. All of these factors combine to reduce the device efficiency, and therefore the potential electricity that can be produced by a solar cell.
Thus, there is a need in the art for a solar cell architecture that overcomes the above difficulties.